1. Field of the Invention
The field of the invention is that of anti-PMD systems, the purpose of which is to reduce the polarization mode dispersion (PMD) of an optical fiber during the fiber drawing process. PMD is a problem in an optical fiber because it degrades the signal propagating in the optical fiber. This problem is becoming increasingly severe as the bit rates in optical fiber communication networks increase.
2. Description of the Prior Art
A prior art anti-PMD system known as the SZ system is based on the use of a flat pulley oscillating in rotation. It is described in U.S. Pat. No. 5,298,047, for example, herein incorporated by reference, and in European patent EP 744636. The SZ system includes a flat pulley. A flat pulley is one whose peripheral external surface is cylindrical. The SZ system is positioned at the base of the fiber drawing tower, in the plane through which the optical fiber passes, the optical fiber executing an elbow bend at the flat pulley. The plane through which the optical fiber passes is also known as the fiber drawing plane. The flat pulley, rotating about its rotation axis, also oscillates about the vertical fiber drawing axis, which is coincident with its oscillation axis, and symmetrically with respect to the fiber drawing plane. V-shaped pulleys upstream of the oscillating flat pulley and guide fingers downstream of the oscillating flat pulley guide the optical fiber in the fiber drawing plane. Oscillations of the pulley, alternately from right to left and then from left to right, apply a torsion torque to the optical fiber, which is already covered with its secondary coating. A drawback of this prior art system is that it produces optical fibers whose PMD is still too high.
The invention proposes an improved anti-PMD system which further reduces the PMD of optical fibers during fiber drawing. To this end, the invention proposes an anti-PMD system based on using a pulley oscillating in rotation and which is convex in order to increase the ratio between, firstly, the angle between the optical fiber in contact with the pulley and the normal to the rotational axis and to the oscillation axis of the pulley and, secondly, the angle between the optical fiber drawing plane and the normal to the rotation axis and to the oscillation axis of the pulley, to increase the torsion torque applied to the optical fiber to reduce the PMD of the optical fiber. If the ratio between, firstly, the angle between the optical fiber in contact with the pulley and the normal to the rotation axis and the oscillation axis of the pulley and, secondly, the ratio between the optical fiber drawing plane and the normal to the rotation axis and the oscillation axis of the pulley increases, for a given amplitude of oscillation of the pulley, the angle between the optical fiber in contact with the pulley and the normal to the rotation axis and the oscillation axis of the pulley increases, which causes the optical fiber to twist more, thereby increasing the torsion torque applied to the optical fiber.